This invention relates generally to thermal transfer printing, and more specifically the present invention is directed to the preparation of transparencies with certain selected overcoating compositions enabling their compatibility with donor sheets used in thermal transfer printing systems. In one specific embodiment of the present invention there are provided polymeric coatings for transparencies enabling the acceptable thermal transfer of images from various donor sheets, particularly those selected for use in thermal transfer printing systems. Additionally, the coatings of the present invention can be selected for various substrates that are utilized in ink jet printing apparatuses. Moreover, the transparencies prepared in accordance with the process of the present invention can be used for receiving characters from typewriters with single strike correctable ribbons.
In thermal transfer printing, there is used an ink donor film consisting generally of a thin substrate coated with a waxy ink. In operation, the inked portion of the donor film is placed in contact with a plain paper receiver sheet followed by directing heat from a thermal printhead to the film side of the donor, causing ink to melt and wet the receiver sheet. Subsequent separation of the sheets provides for the transfer of ink to the image areas of the plain paper. Various substrates are of interest for the ink donor film including highly calendered condenser papers with thicknesses of from about 10 to about 20 microns and polyester (Mylar) films. Thermal printers utilizing such materials are useful in a variety of applications including facsimile, printer plotters, and computer output printers. As office automation increases, low cost thermal printers appear to be prime candidates for communicating terminals, allowing for office quality hard copy inputs. The main advantages of these printers are reliability, clean operation, compactness, speed and low cost.
Generally, the thermal printers can be classified as direct and transfer. In the direct system, a paper having a thermally sensitive coating, either a wax or an organic metal compound is selectively heated causing color changes in the coating. In the transfer type of printer, a donor ribbon loaded with a marking material, typically a heat sensitive ink, is transported intermediate to a thermal printhead and a plain paper recording sheet. The printhead is electrically activated to selectively apply heat to the donor sheet thereby causing melting and transfer of portions of the marking material onto the paper in image configuration.
Disclosed in U.S. Pat. No. 4,273,602, are heat sensitive recording materials comprised of a support sheet of a thickness of from 5 to 40 microns having thereon a heat sensitive transfer layer with a phenolic material, a colorless or precolored material which reacts with the phenolic material to form color upon application of heat, and a heat fusible material having a melting point of 40.degree. C. to 150.degree. C., with an image receiving sheet superimposed on the surface of this layer. It is indicated in this patent that heat sensitive transfer layers can be formed from waxes or resins of low molecular weight, with colored dyes dispersed therein, however, apparently there are problems associated with such a method in that part of the layer transfers to ordinary paper causing undesirable staining and a decrease in contrast between letters and the background. Accordingly, the recorded letters cannot be easily read.
Also known is the preparation of transparencies by electrostatic means. There is thus disclosed, for example, in U.S. Pat. No. 4,370,379 a method for preparing an original for projection according to electrophotographic processes. More specifically, it is indicated in this patent that the conventional method for preparing a projection original for an overhead projector, (a transparent sheet), according to electrostatic photography comprises transferring a toner image formed on a photosensitive plate onto a stretched polyester film, and fixing the transferred toner image on the film by heat. Various plastic films can be used for this purpose including a biaxially stretched polyester film. It is further indicated in this patent that the transfer film selected for electrostatic photography is comprised of a film substrate which is transparent such as a biaxially stretched polyethylene terephthalate film including Mylar films. The present invention relates to the formation of similar transparencies which contain certain coatings thereover, and thus can be used in thermal transfer printing systems.
Moreover, there is disclosed in U.S. Pat. No. 4,234,644, a composite lamination film for electrophoretically toned images deposited on a plastic dielectric receptor sheet, comprising in combination, an optically transparent flexible support layer, an optically transparent flexible intermediate layer of a heat softenable film applied to one side of the support layer, wherein the intermediate layer possesses good adhesion to the support layer. It is indicated in this patent that the support layer 11 can be prepared from various suitable substances including polycarbonates, polysulfones, polyethylene terephthalates, Mylars, and the like.
While the above-described transparencies are suitable for their intended purposes, there remains a need for transparencies which are compatible with donor sheets enabling the acceptable thermal transfer of images thereto. Moreover, there remains a need for coatings for transparencies enabling their effective and efficient use in allowing images electrostatically formed to be transferred thereto. Also, there continues to be a need for coatings for transparencies which can be generated in ink jet printing systems.